In the disclosure of the present invention reference is mostly made to the treatment of diabetes by injection or infusion of insulin, however, this is only a preferred use of the present invention.
In order to permit a patient to administer a proper dose, various mechanical injection devices have been proposed which offer additional electric circuitry for measuring and displaying the size of a dose which will be administered during an injection procedure. Examples of such devices are shown in WO02/064196 and in WO02/092153.
Some mechanical injection devices are equipped with an energy accumulating member such as a spring for the purpose of accumulating mechanical energy during a pre-injection procedure, e.g. by straining a spring during a dose setting procedure, to enable the accumulated energy to drive a drive member during the subsequent injection procedure so as to expel a fixed or an individually set dose. WO2008/037801 disclose such devices wherein additional electric circuitry is provided for detecting the amount of a set dose or the amount of an injected dose.
Several different unwanted conditions is likely to occur during operation of mechanical injection devices, such as the occurrence of a blocked injection needle or the patients usage of an injection device which has not been properly primed or purged prior to making an injection. Such conditions may present serious hazards if they are not acknowledged by the user and appropriate action is taken. In mechanical injection devices where the user exerts a force upon the drive mechanism for forcing the plunger of the cartridge forward during the complete injection procedure, the user may experience some perceptive feedback from the drive mechanism during operation thereof. Generally, such feedback provide some degree of alertness as to eventual unwanted conditions. However, when using spring assisted injection devices, such as the ones shown in WO2008/037801, the mechanical perceptive feedback may be rather limited.
The documents US 2003/0009133, US 2003/0073954, U.S. Pat. No. 6,340,357, WO 03/099357 and WO2006/116997 all relate to motorized injection or infusion apparatuses where motor driving circuitry is used for controlling the motor drive. The motor driving circuitry can be easily adapted to monitor the drive mechanism during operation to allow for detection of specific conditions and for alerting the patient of the detected condition. In WO 2007/107558 a cartridge sensor system is disclosed, which enables detection of the axial position of a plunger inside the cartridge and by using additional sensors in an associated medication delivery device a needle blockage condition may be detected.
It is desirable following transcutaneous injection of a medicament to wait a short time before withdrawing the needle of the injection device from the skin of the patient. Generally, medical doctors instruct their patients in that they shall, subsequent to carrying out an injection procedure, wait 5-10 seconds, for larger doses even longer, before retracting the needle from the skin. Commonly it is acknowledged that this is necessary because the injected medicament takes a few seconds to disperse locally from the injection site within the body of the patient. If the needle is withdrawn too soon after the medicament has been injected, there is a risk that at least some of the injected medicament may escape through the needle wound and not be dispersed.
Unfortunately, this problem is particularly acute where a patient uses a medicament injection apparatus to self-administer a medicament. A user may become impatient and withdraw the needle before the medicament has dispersed.
In order to alleviate this problem, WO patent application No. 97/30742 discloses an injection device which provides an electronic representation of a recommended waiting period after completion of an injection movement for advising the patient to keep the needle inserted in the skin during the recommended waiting period.
The required waiting period is a time contributing to the overall time needed by a diabetic patient to treat the condition and should generally be minimized. Further, as the waiting period is a passive time (no action needed or allowed by the patient) it is likely to psychologically feel like a larger percentage of the overall time used than the real percentage. Finally, a simple and unintelligent maximum fixed waiting time scheme is likely to increase the risk that some patients become impatient and withdraw the needle prematurely.
To implement the abovementioned principle of recommending a longer waiting period when a large dose has been injected as compared to injection of smaller doses, WO patent application Ser. No. 03/057,283 discloses a medicament injection apparatus where the point in time of actuation of a signal means for signalling the end of the waiting period is made dependent of the amount of the medicament injected. Even though that such a system may be well received by some patients, this simple corrective action may lead to incorrect guidance of the user, having the effect that some users may experience fluid leakage around the needle wound and dripping from the needle when the needle is retracted from the skin.